boost::asio::ip::tcp::socket is connected?

If you are sure that the remote socket has not sent anything (e.g. because you haven't sent a request to it yet), then you can set your local socket to a non blocking mode and try to read one or more bytes from it.

Given that the server hasn't sent anything, you'll either get a asio::error::would_block or some other error. If former, your local socket has not yet detected a disconnection. If latter, your socket has been closed.

Here is an example code:

#include <iostream>
#include <boost/asio.hpp>
#include <boost/asio/spawn.hpp>
#include <boost/asio/steady_timer.hpp>

using namespace std;
using namespace boost;
using tcp = asio::ip::tcp;

template<class Duration>
void async_sleep(asio::io_service& ios, Duration d, asio::yield_context yield)
{
  auto timer = asio::steady_timer(ios);
  timer.expires_from_now(d);
  timer.async_wait(yield);
}

int main()
{
  asio::io_service ios;
  tcp::acceptor acceptor(ios, tcp::endpoint(tcp::v4(), 0));

  boost::asio::spawn(ios, [&](boost::asio::yield_context yield) {
    tcp::socket s(ios);
    acceptor.async_accept(s, yield);
    // Keep the socket from going out of scope for 5 seconds.
    async_sleep(ios, chrono::seconds(5), yield);
  });

  boost::asio::spawn(ios, [&](boost::asio::yield_context yield) {
    tcp::socket s(ios);
    s.async_connect(acceptor.local_endpoint(), yield);

    // This is essential to make the `read_some` function not block.
    s.non_blocking(true);

    while (true) {
      system::error_code ec;
      char c;
      // Unfortunately, this only works when the buffer has non
      // zero size (tested on Ubuntu 16.04).
      s.read_some(asio::mutable_buffer(&c, 1), ec);
      if (ec && ec != asio::error::would_block) break;
      cerr << "Socket is still connected" << endl;
      async_sleep(ios, chrono::seconds(1), yield);
    }

    cerr << "Socket is closed" << endl;
  });

  ios.run();
}

And the output:

Socket is still connected
Socket is still connected
Socket is still connected
Socket is still connected
Socket is still connected
Socket is closed

Tested on:

Ubuntu: 16.04
Kernel: 4.15.0-36-generic
Boost: 1.67

Though, I don't know whether or not this behavior depends on any of those versions.